1. Field of the Invention
This invention relates to a film thickness measurement device and method.
The entire contents of Japanese Patent Application No. 2005-297339, filed on Oct. 12, 2005, are incorporated herein by reference.
2. Description of the Related Art
One method of measuring the thickness of an oil film within an engine cylinder during operation entails using a bridge circuit to measure the electrostatic capacitance across the gap between the piston ring and cylinder (more specifically, a sensor electrode provided on the piston ring sliding surface of the cylinder liner), in which the oil film is formed, and calculating the film thickness based on this electrostatic capacitance (see for example Japanese Unexamined Patent Application No. 2003-214810).
In the electrostatic capacitance measurement method using this bridge circuit, a bridge circuit is configured with the gap between the piston ring and the sensor electrode as one capacitor (the measurement capacitance), and with three other reference capacitors; by adjusting the reference capacitors such that the bridge balance condition is satisfied according to the electrostatic capacitance changes of the measurement capacitance, the electrostatic capacitance of the measurement capacitor is determined.
Moreover, a resonant bridge circuit method is another electrostatic capacitance measurement method using a bridge circuit. In this resonant bridge circuit method, the measurement capacitor is positioned in one of the arms of a bridge circuit using resistors, and by varying the power supply frequency while detecting the diagonal potential difference of the bridge circuit, the frequency at which the diagonal potential difference is maximum, that is, the resonance frequency, is determined, and based on this resonance frequency the electrostatic capacitance of the measurement capacitor is computed.
However, when a bridge circuit is used as in the above methods of the prior art, changes in the electrostatic capacitance of the measurement capacitor are very small, so that it has been necessary to position the bridge circuit in the vicinity of the measurement point, that is, the cylinder, due to noise considerations. In this case, the measurement capacitor is exposed to high temperatures, so that the electrostatic capacitance of the measurement capacitor changes due to temperature dependence, and error occurs in the measured results for the electrostatic capacitance of the measurement capacitor; consequently there is the problem that measurement errors occur in the measured thickness of the oil film.
Moreover, when the resonant bridge circuit method is used, by using resistors with a comparatively low temperature dependence as circuit elements, measurement errors due to temperature can be alleviated; but because the power supply frequency must be varied to determine the resonance frequency, high-speed measurement (realtime measurement) is difficult, and so there are the problems that the measurement device configuration is complex and costly, and in addition that time is required to perform judgments.